I have someone who installed a ceiling fan with light and I told them how to hook it up but they want to work the light from the switch and have the ceiling fan going when the switch is off. I told them they would need 2 switches instead of one which powers the fan and light and can only be controlled by pulling the chains to turn them off or on.
How can they replace the single switch to a double switch-one to control the fan and on for the light?

Installing a second switch isn't the problem here, it's running another wire to the fan that will allow the second switch to work. If it's in conduit, not a big problem. But if it's cable in the walls, another wire would have to be run with 3 conductors and a ground.

We need to know what wires they have in the ceiling.

Ceiling fan wiring - Ceiling Fans N More (http://www.ceiling-fans-n-more.com/ceiling-fan-wiring.php)